I treat all new fish with Prazi in QT either during or after TT.
With fish that are eating well during TT I treat them during transfers 2 and 4. No water change needed because you are transferring every 3 days anyways.
If I wait until in QT, I usually dose Prazi on day 1 and then do a WC on day 5 and redose on day 6. I believe there are some that don't bother with a WC but i prefer to do one because you are not running a skimmer during treatment and it helps avoid a build up for nutrients in the tank that could lead to an ammonia spike, especially if in a smaller QT. I prefer 3 treatments as opposed to the standard 2 so I dose on days 1, 6, and 11 and do WCs on days 5, 10, and 15 and after last WC I turn back on skimmer and add carbon.